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23 Effect of synchronization error between force and position data in moment calculations
Abstracts
in the computer model: pelts, femur, tibia. patella, and foot. The hip and ankle were modeled as frictionless rcvolute jomts; the knee kinematics incorporated rolling and sliding motions that depended on knee flexion angle, as described by Delp er al. (1990). The translation and tilt of the pelvis were prescr&ed as functions of time, leaving flexiotiextension of the hip, knee. and ankle as the only degrees of freedom in the model. Initial conditions for the simulation were prescribed according to experimental measurements of the toe-off gait kinematics. We used mathematical descriptions of musculotendon actuators (Zajac. 1989) to represent the actions and force-generating capabilities of twelve lower-extremity muscles in the simulation. Muscle forces were calculated throughout the simulatmn as functions of fiber length, fiber velocity, and activation. Muscle excitation. approximated from electromyographic (EMG) data measured by Perry (1992) during normal gait, determined activation through first-order activation dynamics as described by Zajac (1989). The ability of the model to reproduce normal swing was confirmed bv commuine simulated sa.aittal plane ioint kinematics to those measured during nornul gait.‘F”rth;r vasdatio” of the 6mulation was accomplished by comparing the sum of the ioint moments aoolied bv the model’s muscles to the moments calculated by Winter (1991) fro& ~onnal kinem& gait &a using a” mverse-dynamic model. Simuiarion nnalyses: To Investigate the role of
El
Effect
of Synchronization Error Between Force and Position Data in Moment Calculations E.J.Alexander, D.E.Hmwitz, T.P.Andriacchi, Department of Orthopedic Surgery Rush-Presbyterian-St Luke’s Medical Center, Chicago, IL
Introduction Accurate calculation of the external moments acting o” the lower limb joints requires both accurate video and force data as well as accurate temporal synchronization between the two. The purpose of this study was to test the hypothesis that eve” a one frame synchronization error causes a StatisticalIy significant change in the external moments at the ankle, knee, and hip during gait. Materials and Methods The joint kinetics of ten adults (3 female, 7 male) with no significant musculoskeletal involvement were studied. The subjects (age: 27 f 7 years, height: 174.4 k 7.4 cm. and
111
weight: 72.0 + 11.9 kg) perfornxd level walkmg t” a gait analysis clinic. Subjects walked at three self selected speeds of slow, normal, and fast and data were collected with a three dimensional optoelectronic system and force plate. A representative trial with a speed closest to 1 m/xc was analyzed. Force data were acquired at 120 Hz and position data at 60 Hz. The position data was then linearly interpolated to match the I20 Hz force data. Inverse dynamics were wed to calculate the external moments at each joint center from the force and position data in conjunction with the limb segment inertia properties (1). A software modification was then used to move the force data forward one frame with respect to the position data (the force-leading data set) and then to move the force data backward one frame with respect to the position data (the force-trailing data set). Significant differences between the correctly synchronized data set and the shifted data sets were tested for with a paired I test and a significance level of a < 0.05. All moment values are external moments and are normalized percent body weight times height. Results Even a one frame synchronization error caused a significant change in the sagittal plane joint moments @ < ,001). The largest effect was seen in the hip extension moment (Figure 1) and the knee flexion moment (Figure 2). The external knee @ = .Ol) and hip (p = .02) adduction moments also showed a significant change from a single frame shift synchronization error.
!
-I
I T
3
I
L Figure 1: A one frame synchronization extension moment (p ct ,001).
error caused a” 18% change
in the external
hip
r
I
Figure flcxion
2: A one frame synchronization moment (p < ,001).
,“Lh
error caused
a” 11% change
in the external
knee
Discussion The results of the study indicate that synchronization errors as small as l/120’ of a second can have a significant effect on the lower limb moment calculation, with the maximum knee flexion moment being particularly sensitive to synchronization. It is also clear that at a lower sampling rate a single frame shift would result in even greater error.
1) Andriacchi
et al., NATO
Acknowledgments:
The “Poker”
NIH
ASI SERIES AR 20702.
Test: A Spot Check
E!l Musgrave
Park Hospital,
NlH
References E: No. 93, pp 83-102,
1985.
AR 39432.
to confirm
the accuracy
Richard Baker, PhD Stockman’s Lane, Belfast
of K&tic
Gait Data
BT9 7JB, N.Ireland
Introduction Kinetic data such as joint moments and powers are now often used in clinical gait analysis Such data are complex and variable from patient to patient and loss of accuracy in any of the measuring systems used will not always be obvious from examination of the data. This paper presents a simple spot check for confirming the accuracy of kinetic measurements.
in the computer model: pelts, femur, tibia. patella, and foot. The hip and ankle were modeled as frictionless rcvolute jomts; the knee kinematics incorporated rolling and sliding motions that depended on knee flexion angle, as described by Delp er al. (1990). The translation and tilt of the pelvis were prescr&ed as functions of time, leaving flexiotiextension of the hip, knee. and ankle as the only degrees of freedom in the model. Initial conditions for the simulation were prescribed according to experimental measurements of the toe-off gait kinematics. We used mathematical descriptions of musculotendon actuators (Zajac. 1989) to represent the actions and force-generating capabilities of twelve lower-extremity muscles in the simulation. Muscle forces were calculated throughout the simulatmn as functions of fiber length, fiber velocity, and activation. Muscle excitation. approximated from electromyographic (EMG) data measured by Perry (1992) during normal gait, determined activation through first-order activation dynamics as described by Zajac (1989). The ability of the model to reproduce normal swing was confirmed bv commuine simulated sa.aittal plane ioint kinematics to those measured during nornul gait.‘F”rth;r vasdatio” of the 6mulation was accomplished by comparing the sum of the ioint moments aoolied bv the model’s muscles to the moments calculated by Winter (1991) fro& ~onnal kinem& gait &a using a” mverse-dynamic model. Simuiarion nnalyses: To Investigate the role of
El
Effect
of Synchronization Error Between Force and Position Data in Moment Calculations E.J.Alexander, D.E.Hmwitz, T.P.Andriacchi, Department of Orthopedic Surgery Rush-Presbyterian-St Luke’s Medical Center, Chicago, IL
Introduction Accurate calculation of the external moments acting o” the lower limb joints requires both accurate video and force data as well as accurate temporal synchronization between the two. The purpose of this study was to test the hypothesis that eve” a one frame synchronization error causes a StatisticalIy significant change in the external moments at the ankle, knee, and hip during gait. Materials and Methods The joint kinetics of ten adults (3 female, 7 male) with no significant musculoskeletal involvement were studied. The subjects (age: 27 f 7 years, height: 174.4 k 7.4 cm. and
111
weight: 72.0 + 11.9 kg) perfornxd level walkmg t” a gait analysis clinic. Subjects walked at three self selected speeds of slow, normal, and fast and data were collected with a three dimensional optoelectronic system and force plate. A representative trial with a speed closest to 1 m/xc was analyzed. Force data were acquired at 120 Hz and position data at 60 Hz. The position data was then linearly interpolated to match the I20 Hz force data. Inverse dynamics were wed to calculate the external moments at each joint center from the force and position data in conjunction with the limb segment inertia properties (1). A software modification was then used to move the force data forward one frame with respect to the position data (the force-leading data set) and then to move the force data backward one frame with respect to the position data (the force-trailing data set). Significant differences between the correctly synchronized data set and the shifted data sets were tested for with a paired I test and a significance level of a < 0.05. All moment values are external moments and are normalized percent body weight times height. Results Even a one frame synchronization error caused a significant change in the sagittal plane joint moments @ < ,001). The largest effect was seen in the hip extension moment (Figure 1) and the knee flexion moment (Figure 2). The external knee @ = .Ol) and hip (p = .02) adduction moments also showed a significant change from a single frame shift synchronization error.
!
-I
I T
3
I
L Figure 1: A one frame synchronization extension moment (p ct ,001).
error caused a” 18% change
in the external
hip
r
I
Figure flcxion
2: A one frame synchronization moment (p < ,001).
,“Lh
error caused
a” 11% change
in the external
knee
Discussion The results of the study indicate that synchronization errors as small as l/120’ of a second can have a significant effect on the lower limb moment calculation, with the maximum knee flexion moment being particularly sensitive to synchronization. It is also clear that at a lower sampling rate a single frame shift would result in even greater error.
1) Andriacchi
et al., NATO
Acknowledgments:
The “Poker”
NIH
ASI SERIES AR 20702.
Test: A Spot Check
E!l Musgrave
Park Hospital,
NlH
References E: No. 93, pp 83-102,
1985.
AR 39432.
to confirm
the accuracy
Richard Baker, PhD Stockman’s Lane, Belfast
of K&tic
Gait Data
BT9 7JB, N.Ireland
Introduction Kinetic data such as joint moments and powers are now often used in clinical gait analysis Such data are complex and variable from patient to patient and loss of accuracy in any of the measuring systems used will not always be obvious from examination of the data. This paper presents a simple spot check for confirming the accuracy of kinetic measurements.